The present invention concerns a locking device for a casing containing pyrotechnic materials. It applies for instance to the reduction of the vulnerability of propulsion devices to external aggressions.
Two types of aggression have safety implications for the storage and use of products containing pyrotechnic charges, in particular propellant devices: aggressions due to overheating, and aggressions of a mechanical type.
In aggressions due to slow or rapid overheating, two phases can be distinguished. The first is constituted by the rise in temperature of a propellant device, for example, up to the self-ignition temperature of the pyrotechnic materials, generally propellant grain. The period between the start of the thermal aggression and the pyrotechnic incident depends on the self-ignition temperature of propellant grain and also on the structural parts surrounding the charge such as collars, thermal protections or inhibitors for instance. Generally the invulnerability specifications lay down a minimum period, for example 3 to 5 minutes, for this self-ignition.
The second phase corresponds to the pyrotechnic incident itself. Neither detonation nor deflagration are acceptable, but if combustion can not be totally avoided a pyrotechnic incident of minimal gravity is sought, that is to say a combustion with neither propulsion nor projection. However, the rise in pressure of gases generated by self-ignition of the pyrotechnic charge, for example propellant grain, leads to a explosion of the casing of the propellant device, thus to a deflagration. In a propellant device, this rise in pressure is indeed very difficult to avoid for the casing of the latter is designed to withstand a very high internal pressure, generally the maximum functioning pressure plus a safety margin, which may be several hundred bars.
In the case of mechanical types of aggression, for instance impacts of bullets or fragments, it is not possible to rely on a local deformation, a penetration for instance, caused by the mechanical aggression, in order to avoid a rise of pressure of the casing if a pyrotechnic incident takes place. Propellant and pyrotechnic products must therefore be chosen which do not detonate easily under impact and the system designed so that any pyrotechnic incident caused by an impact results in neither deflagration nor propulsion.
In order to avoid the incidents mentioned above, so-called "active" protection processes are known to professionals. These processes consist in completely opening up the casing as soon as an accidental incident is discovered. They have the inconvenience of requiring further components such as captors, sources of energy or activators for instance, which significantly increase the complexity of the system. Moreover, their operation must be assured in all accidental environments.